Skip creating variants for disabled OSes

The documentation java_genrule_host states that it creates a single
variant, which would make it work with the single variant fallback
in AddDependency used by the data property, but it actually has a host
and a host-cross windows variant.  Modify osMutator to take the
OS-specific enabled properties into account to skip creating variants
that will immediately be disabled so there is a single variant.

Test: m checkbuild
Change-Id: Ic2daab29f4fa3a3797d7a08348fbfcf1036ec5dc

android/defaults.go

@@ -174,20 +174,22 @@ func (d *DefaultsModuleBase) GenerateAndroidBuildActions(ctx ModuleContext) {
 
 func InitDefaultsModule(module DefaultsModule) {
 	commonProperties := &commonProperties{}
+	enabledProperties := &enabledProperties{}
 
 	module.AddProperties(
 		&hostAndDeviceProperties{},
+		enabledProperties,
 		commonProperties,
 		&ApexProperties{},
 		&distProperties{})
 
+	base := module.base()
 	initAndroidModuleBase(module)
 	initProductVariableModule(module)
 	InitArchModule(module)
 	InitDefaultableModule(module)
 
 	// Add properties that will not have defaults applied to them.
-	base := module.base()
 	defaultsVisibility := &DefaultsVisibilityProperties{}
 	module.AddProperties(&base.nameProperties, defaultsVisibility)